In the United States, and, in fact, throughout the world, there is a large demand for cotton denim garments which have a distinctly faded, partially worn appearance. These garments are referred to as "frosted", "iced", "whitewashed", or "acid-washed". Although cotton denim accounts for the bulk of the "frosted" fabrics, other cotton materials, such as different types of twills and cotton corduroys are also subject to frosting.
It has been known for many years to use abrasive materials, such as pumice, to "stone-wash" dyed cotton twills to create a preworn appearance. More recently, this "worn" or "distressed" look has become particularly fashionable in apparel made from denim or similar cotton fabric, which has usually been dyed a bluish to black color. Earlier processing consisted primarily of dry tumbling the cotton garments with a quantity of pumice stones in commercial washing machines so that part of the dye was mechanically removed from the fabric. However, a non-abrasive bleaching process for "bluejeans" has been proposed (U.S. Pat. No. 4,218,220). Currently, the "bluejean" industry's preference is for methods which include mechanical abrasion in combination with chemical bleaching.
In one industry practice, natural pumice stones, which are porous, are soaked in an aqueous solution of an oxidizing-type bleaching agent. Both sodium hypochlorite and potassium permanganate have been used. There is a preference for potassium permanganate since the hypochlorite tends to damage the fabric. Any staining from residual manganese dioxide can be removed with a neutralizing agent, such as sodium bisulfite. Permanganate is also advantageous in that it avoids the production of irritating fumes which can result from a chlorine-liberating bleaching agent.
In the use of natural pumice stones pre-soaked in an aqueous solution of a bleaching agent, several practical disadvantages have been encountered. Even though the stones are drained of excess solution, they can continue to release the bleaching solution by seepage during storage. Further, the initial contacting of the stones with the moist denim garments can result in overbleaching. As the treatment continues the degree of bleaching decreases. Such irregular bleaching can result in a streaked, unattractive appearance. Moreover, the bleach solution is substantially exhausted with each use of the stones, and they must be resoaked frequently.
Several improvements have been proposed. In one procedure which is being used commercially to some extent, the natural pumice stones are pre-impregnated under pressure/vacuum conditions, the details of which are not known. This preparation treatment may provide for greater degree of impregnation, and more use of the stones without recharging. However, these stones do continue to exude solution on standing, and the initial use of the stones can result in spotty bleaching which is generally undesirable.
Another alternative which has also received some degree of commercial use is to employ a loose mixture of a bleaching agent with an inert but somewhat abrasive filler, such as, for example, potassium permanganate powder and fine quartz sand or other siliceous material. The frosting effect obtained from such a free-flowing mixture is rather "flat" and is considered to be less attractive.
Potassium permanganate and other bleaching agents have been encapsulated or adsorbed on support materials or embedded in formed bodies for other purposes. See, for example, U.S. Pat. Nos. 3,535,262, 4,279,764, 4,460,490, 4,665,782, 4,657,784, and 4,711,748. German Patent No. 2,311,964 describes the preparation of a product for decontaminating radioactive waste containing manganese dioxide (MnO.sub.2) in Plaster of Paris (gypsum). A slurry is formed from manganese sulfate (MnSO.sub.4) and potassium permanganate (KMnO.sub.4) and gypsum which is cast into blocks. The MnSO.sub.4 and KMnO.sub.4 react in the slurry to form the MnO.sub.2, which is dispersed throughout the gypsum mass and removes radionuclides by adsorption.